The Sky This Week, 2015 May 19 - 26

The Moon visits Venus & Jupiter, Saturn reaches opposition.

Lunar craters Theophilus and Cyrillus, imaged on 2013 September 24

The Moon returns to the evening sky this week, waxing through the crescent phases as she passes the bright planets Venus and Jupiter. First Quarter occurs on the 25th at 1:19 pm Eastern Daylight Time. Luna may be found near Venus on the evenings of the 20th and 21st, she passes five degrees to the south of Jupiter in the evening of the 23rd.

The return of the Moon offers another chance to explore our only easily accessible alien landscape. This week marks the 46th anniversary of the flight of Apollo 10, the “dress rehearsal” for the first Moon landing. On the evening of the 23rd the area that the Apollo 10 astronauts scouted for that historic landing will be visible under the same lighting conditions that they had on that pioneering flight. Needless to say that particular area of the Moon’s surface is one of the smoothest on the entire surface, but nearby the Apollo 11 landing site are a number of striking formations. My favorite among these are a pair of impact craters that are featured prominently just south of the lunar equator on the northwest “shore” of Mare Nectaris, the smallest of the near-side lunar “seas”. The most prominent of these craters is Theophilus, a sharp-rimmed formation with a prominent central peak. Its rim cuts into a similar-sized crater, Cyrillus, indicating that Theophilus is the “fresher” of the two craters. Both features have diameters of about 100 kilometers (60 miles). They can be easily seen in binoculars when near the terminator as they will be on the 23rd, and small telescopes will begin to reveal the terraced walls and complex central peaks of Theophilus. A six-inch or larger telescope and good seeing conditions will show many of the secondary impact craterlets that litter the floor of Mare Nectaris as a result of the impact that formed Theophilus around one billion years ago. Over the course of the next several nights, the lunar terminator unveils more and more of the Moon’s battered southern face, mute testament to the chaos that ruled when the solar system was in its formative stages. Take the time to give our nearest neighbor a good long look this week.

The Moon washes out the fainter stars of the spring constellations as the week progresses, but the signature stars of the season should still be visible. The Big Dipper crosses the meridian in the northern sky as evening twilight ends; looking to the south you’ll find Leo the Lion just west of the meridian. In the east the bright rosy glow of Arcturus dominates the view. If you follow the “arc” of the Bid Dipper’s “handle” to Arcturus, continue the imaginary line to the south to “speed on to Spica”, the brightest star in the constellation of Virgo. Finally, as full darkness settles in during the late evening hours, you should spot the blue glint of the bright star Vega, one of the signature stars of summer.

The twilight hour is dominated by the bright glow of Venus, which is now close to her greatest elongation east of the Sun. The dazzling planet continues to press eastward against the background stars and is setting her sights on Jupiter, who pops out of the twilight glow shortly after Venus. The two planets are just over 30 degrees apart at the beginning of the week, but that gap is trimmed to 25 degrees by week’s end.

Jupiter continues to lose ground to both Venus and the Sun, but it’s still worth turning the telescope toward him for an interesting view of the most intense weather in the solar system. The planet’s Great Red Spot rotates across the disc on the evening of the 20th. At the same time you may notice two dark spots crossing as well. One of these is the moon Callisto, whose dark surface looks like a moon’s shadow. The other spot is the shadow of Io, which will be just west of the planet’s limb. You’ll get another chance to see the Red Spot on the evening of the 25th.

Saturn reaches opposition on the night of the night of the 22nd. The planet is moving along the southern reaches of the Ecliptic during this apparition, and may be found ensconced among the stars that form the “head” of Scorpius. Saturn rises at sunset and sets near sunrise during the course of the week, but his far southerly declination doesn’t bring him into a good position for telescopic viewing until the late evening. Saturn’s disc is about half the apparent size of Jupiter’s, and compared to the giant planet it’s downright bland. This is more than made up for by the planet’s spectacular rings, though, so take some time to track him down to take a look.